Trona ore used for the manufacture of soda ash consists mainly of sodium sesquicarbonate and insoluble matter in the form of shale. The shale contains various constituents such as organic kerogeneous matter (about 0.2% as C); dolomite (about 5.5%); and silica bearing materials (quartz about 1.1%, feldspar about 3%, clay about 0.5%).
In the production of soda ash the ore is calcined at about 180.degree. C. to decompose the sodium sesquicarbonate to sodium carbonate. EQU 2(Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O).fwdarw.3Na.sub.2 CO.sub.3 +5H.sub.2 O+CO.sub.2
During calcination, a part of the silica contained in the ore is converted to soluble silicates, and the organic kerogeneous matter is converted into soluble organic compounds. The calcined ore is dissolved in water or dilute soda liquor to give a saturated solution of 30% Na.sub.2 CO.sub.3 and the soluble impurities mainly silica and organics. Other minor soluble impurities consist of NaCl and Na.sub.2 SO.sub.4. The insoluble material is removed by settling and filteration, and sodium carbonate monohydrate is recovered by evaporative crystallization. The concentration of soluble impurities in the mother liquor increases to .about.10,000 ppm SiO.sub.2 (basis Na.sub.2 CO.sub.3) and .about.4000 ppm C (basis Na.sub.2 CO.sub.3). To avoid contamination and deterioration of crystal shape by the impurities, a portion of the liquor must be purged. This results in a loss of about 10% of the soda values. The purge liquor along with other waste streams from the plant is generally stored in ponds which use up large areas of land.
While various methods have been proposed to provide an economically viable system to recover the trona process liquor and, to minimize waste accumulation, none has been heretofore satisfactory.
One of the methods for the recovery of soda values from purge liquor consists in cooling the liquor to below 35.degree. C. to crystallize out the Na.sub.2 CO.sub.3 in the form of decahydrate crystals. The decahydrate rejects most of the impurities and can be redissolved in the crystallizer liquors. However, this scheme requires additional capital expense for sizable equipment to separate the decahydrate crystals. Also, such sizable equipment must be operated within close limits which is difficult.
In another method, as disclosed in U.S. Pat. No. 4,044,097, the purge liquor is treated with MgO, Al.sub.2 O.sub.3, bauxite, insolubles from calcined trona ore or mixtures thereof, evaporated to dryness and heated to 300.degree.-600.degree. C. to insolubilize the silica and to decompose the organic content. The calcined mixture is thereafter leached with water or with a solution of sodium carbonate to recover the soda ash values. This process is relatively energy intensive because it requires the solution to be evaporated to dryness and further requires heating the residue to a high temperature to decompose the organic constituents.
It is thus apparent that a need exists for a more effective and economical method for deriving pure anhydrous sodium carbonate crystals from the impure process liquors.